Permanently connecting a set of conductive tracks on a substrate with a co-operating set on a printed circuit

ABSTRACT

A flexible printed circuit is secured with adhesive to a glass or plastics substrate. Electrical connection between the tracks of the printed circuit and co-operating tracks on the substrate is made by way of conductive fibers with which the adhesive is loaded in such a proportion that no bridging electrical contact is established between laterally adjacent tracks.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to making permanent electrical connection betweena set of conductive tracks on a substrate and a co-operating set on aprinted circuit. A particular application of this invention is in themaking of edge connection to display devices such as liquid crystaldisplays.

2. Description of the Prior Art

Liquid crystal displays need some kind of connector to make contact withterminal pads on one or perhaps both of the substrates defining theliquid crystal layer. Usually these pads are of the same material as theelectrodes of the display and are transparent, typically being made ofindium tin oxide. A display cell may typically have between 30 and 400such pads. Provided that they are not too numerous, and that they arenot too fine, connections with these pads can sometimes be made usingpush-on connector sockets similar to those developed for making edgeconnection with printed circuit board. However, it is often found thatthis approach is unsatisfactory because of the frequency with which opencircuit faults occur; moreover, there is the added disadvantage that thesockets tend to be relatively bulky. Finally, it is not really practicalwhen the pitch of the pads is significantly less than 1 mm.

An alternative approach, particularly when the pads are relativelynumerous and closely spaced, has been to make electrical connectionbetween the pads and a co-operating set of conductive tracks on a pieceof printed circuit by means of an elastomeric strip which iselectrically non-conductive in the direction of the longitudinal axis ofthe strip, but is electrically conductive in planes normal to this axis.Such strip may be constructed by forming a laminate of alternate layersof electrically conductive and electrically insulating elastomericmaterial, with the layers extending across the strip so that theconduction of electricity in the longitudinal direction is preventedwhile that in transverse direction is permitted. This approach suffersfrom the disadvantage that the laminated elastomeric strip is relativelyexpensive and requires the use of some form of permanent clamping meansto hold the strip in permanent compression. Such clamping means tends tobe relatively bulky. The use of a fibre loaded adhesive is described inthe specification of U.K. patent application No. 2 034 095 A, in whichcarbon fibres embedded in an adhesive matrix material are used not onlyto provide electrical connection between electrodes on the front andrear surfaces of a liquid crystal display cell, but also to providespacers that will determine the spacing between the substrates carryingthose electrodes; and hence, set a precise value to the thickness of theliquid crystal layer.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a method of makingelectrical connection between a set of conductive tracks on a printedcircuit and co-operating conductive tracks on a glass or plasticssubstrate, which method comprises interposing between the printedcircuit and the substrate an adhesive including a dispersion in anelectrically insulating adhesive matrix material of electricallyconductive fibres of substantially uniform diameter and a predeterminedlength not significantly greater than the minimum separation of thetracks in the region covered by the adhesive, which length is such thatno pair of tracks on the printed circuit is electrically bridged by thefibres, and urging the printed circuit and the substrate together toelectrically connect the tracks with the fibres.

The invention also resides in articles incorporating a set of electricalconnections between a set of conductive tracks on a glass or plasticssubstrate and co-operating tracks on a printed circuit when suchconnection has been made by the manner set forth in the precedingparagraph. The invention finds particular application when the pitch ofthe tracks is 1.0 mm of less.

It will be appreciated that for the purposes of making electricalconnection with a printed circuit, the diameter of the fibres; andhence, the thickness of the adhesive layer, is not critical, and thus awider variation of fibre diameter can be tolerated for this purpose thanfor the purpose set out in the patent specification referred to above.

It will also be appreciated that the length of the fibres is liable tobe more critical for the present application particularly in instanceswhere the terminal pads are closely spaced. In this context, it mighthave been thought that it would be essential to avoid the use of fibreshaving a length equal to or exceeding the minimum spacing betweenadjacent pads. However, it has been found that this is not the casebecause the surface of the printed circuit is not flat, but hasappreciable channels whose edges are defined by the edges of theconductive tracks. When the printed circuit is initially pressed againstthe substrate, the last part of the excess uncured adhesive is squeezedout along these channels with the result that the fibres tend in theseregions to become preferentially aligned along the direction of thechannels, this effect being more pronounced in the case of the longerfibres.

BRIEF DESCRIPTION OF THE DRAWINGS

There follows a description of a method embodying the invention in apreferred form of making electrical connection between a set of terminalpads on a liquid crystal display cell and a set of co-operatingconductive tracks on a piece of flexible printed circuit board. Thedescription refers to the accompanying drawings wich show, viewed intransmission, the orientation of fibres in the space between pairs ofconductive tracks of a flexible printed circuit secured with theadhesive to glass substrates provided with co-operating transparentconductive tracks of indium tin oxide.

FIG. 1 is a photomicrograph showing a liquid crystal display cell havingpads disposed at a pitch of 1.27 mm secured to a printed circuit inaccordance with the present invention using an adhesive comprising 5percent by weight of carbon fibres.

FIG. 2 is a photomicrograph showing a similar liquid crystal displaycell and printed circuit assembly in which the adhesive comprises 3percent by weight of carbon fibres.

FIG. 3 is a photomicrograph showing a similar liquid crystal displaycell and printed circuit assembly in which the pads of the cell aredisposed at a pitch of 0.43 mm and the adhesive comprises 5 percent byweight of carbon fibres.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, one face of a glass substrate was provided with anelectrically conductive transparent layer of indium tin oxide; and thislayer was patterned in conventional manner to provide an electrodedesign in which the individual electrodes terminated in a line ofidentical pads arranged side by side in a row with spacing betweenadjacent pads equal to the width of a pad and the pads arranged at apitch of approximately 1.27 mm. This electroded substrate was then usedto form one part of a liquid crystal display cell. Using a specialadhesive, the pads on the substrate were electrically connected withco-operating electrically conductive tracks of copper on a flexibleprinted circuit to form terminal connections for the display cell. Thespecial adhesive consisted of a standard epoxy adhesive in which hadbeen dispersed about 5 percent by weight of carbon fibre derived bygrinding up graphite felt sold by La Carbone (Great Britain) Ltd. underthe designation RVG1000. These fibres have a mean diameter of about 15microns, and the grinding of the fibre in a pestel and mortar providedfragments with lengths predominantly in the range from about 20 micronsto about 150 microns.

When the flexible printed circuit is placed in registry with the pads onthe substrate, the intervening excess adhesive is squeezed out until thecloser approach of substrate and printed circuit is prevented by thetrapping of the carbon fibre fragments between the set of conductivetracks provided by the terminal pads on the substrate and theco-operating set located on the flexible printed circuit. The flexibleprinted circuit was formed by standard photolithographic techniquesapplied to a plastics sheet covered with a layer of copper approximately35 microns thick. Removal of strips of copper so as to define theconductive tracks leaves a surface that is not flat, but has channelsapproximately 35 microns deep between adjacent tracks. The presence ofthese channels means that in the channels the fibres are not compressedfirmly between two surfaces in the way they are in the regions betweenany opposed pair of conductive tracks. A further consequence of thisconfiguring of the surface of the printed circuit is that the squeezingof the final amount of excess adhesive from between the substrate andthe printed circuit produces a flow of adhesive concentrated along thechannels which tends to align the longer carbon fragments with thelongitudinal direction of the channels.

FIG. 1 of the accompanying drawings depicts a photomicrograph of part ofthe adhesive joint so made. This photomicrograph was made byilluminating the joint from the rear. Under these conditions, the copperconductive tracks of the flexible printed circuit show up black and thedrawing shows the nearer edges of two adjacent tracks at 1. Light isable to penetrate through the regions of the flexible printed circuitfrom which the copper has been removed; and hence, the channel 2 betweenadjacent tracks appear white. Here the carbon fibre fragments 3 show upblack against the white background. The photomicrograph also reveals thepresence of a few bubbles in the adhesive which show up as small blackrings 4.

It will be evident from this photomicrograph that with thisconcentration and distribution of fibres and with this track separation,there is essentially no risk of the fibres forming an electricalbridging connection between any track on the printed circuit and eitherof its neighbors. Electrical tests made with probes revealed that theinter-track insulation was essentially unchanged by the use of thecarbon loading in the adhesive. With the adhesive covering about 6 mm oftrack, the use of probes indicated a resistance between an indium tinoxide track on the substrate and the corresponding copper track on theprinted circuit of about 15 ohms. It was suspected, however, that a notinsignificant proportion of this value may be contributed by theresistance present in the connection between the probe and the indiumtin oxide, and by the indium tin oxide film itself which had a surfaceresistance of 15 ohms per square.

Although the foregoing has referred exclusively to the use of carbonloaded adhesive to secure a flexible printed circuit to a glasssubstrate, it will be apparent that the technique is not so restricted.Thus, electrically conductive fibres other than carbon fibres, such asfor instance metal fibres, can be used in the adhesive, the substratecarrying the tracks can be a plastics substrate, and connection may bemade direct to a rigid printed circuit board instead of a flexibleprinted circuit.

Accordingly, a similar piece of printed circuit was cemented with thesame adhesive to a sheet of copper clad plastic using a strip ofadhesive about 6 mm wide; and in this instance, the measured resistancebetween individual tracks on the printed circuit board and the coppercladding was about 0.1 ohms.

FIG. 2 depicts a photomicrograph similar to that of FIG. 1 but in whichthe carbon fibre loading of the adhesive has been reduced from 5 weightpercent to 3 weight percent without producing an noticeable impairmentin the establishment of electrical connection between the co-operatingtracks.

FIG. 3 depicts a photomigraph in which the 5 weight percent loadedadhesive was used to join a piece of printed circuit to an indium tinoxide coated glass substrate for which the pitch of the tracks wasapproximately 0.43 mm (17 thou).

The present invention has been described by referring to a limitednumber of embodiments. Those skilled in the art will recognize thatmodifications other than those specifically mentioned are possiblewithin the spirit of the present invention. Therefore, the scope of theinvention is not limited by the foregoing description, but rather thansolely defined by the following claims.

I claim:
 1. A method of making electrical and mechanical connection between a first set of conductive tracks on a non-conductive flexible printed circuit and a second set of co-operating conductive tracks on a rigid substrate, which method comprises:providing channels on said flexible printed circuit defined by the edges of said first set of conductive tracks, interposing between the printed circuit and the substrate an excess of an adhesive compound including a dispersion in an electrically insulating adhesive matrix material of not more than 10% electrically conductive fibre fragments predominently by weight of a length greater than their mean diameter and not significantly greater than the minimum separation of the tracks in the region covered by the adhesive, said mean diameter being less than the depth of said channels and compressing the printed circuit and the substrate togetherto produce a flow of said adhesive compound concentrated along said channels thereby aligning at least the longer ones of said fibre fragments longitudinally in the direction of said channels and minimizing the risk of any fragments bridging adjacent tracks, to trap at least portions of said fibre fragments between said first and second set of tracks and to electrically connect each track of said first set of tracks with an associated co-operating track of said second set of tracks by means of the conductive fibre portions compressively trapped therebetween.
 2. A method as claimed in claim 1 wherein the pitch of the tracks is 1.0 mm or less.
 3. A method as claimed in claim 1 or 2 wherein the fibres are carbon fibres.
 4. A method as claimed in claim 3 wherein the fibres of the adhesive constitute approximately 5 weight percent of the electrically insulating matrix material of the adhesive.
 5. A method as claimed in claim 1 wherein the fibres are shorter than the minimum separation of the tracks in the region covered by the adhesive.
 6. A method as claimed in claim 1 wherein the substrate constitutes part of a liquid crystal display cell. 